DC/DC converters and dc/ac inverters are used in electronic applications to provide regulated voltages or currents for specialized uninterruptible loads that may be required to be powered by a tightly regulated power source. Such converters and inverters generally operate from an electrical power source that in turn must provide a voltage that is required to lie between well-defined upper and lower voltage limits. For example, a power supply used in a telecommunications or similar application might be required to operate without interruption from a power source that supplies a voltage lying between 43 and 72 volts, and to operate at least for short periods of time for an input voltage as low as 36 volts. However, the power supply might also be required to provide normal output power even if the input voltage falls to zero for a period of time, for example, up to 5 ms. Such uninterruptible operation with no or very little input voltage is often referred to as “holdover,” and usually requires the addition of capacitors of significant size to the circuit to provide the necessary holdover energy. Such holdover capacitors are usually coupled across the input to the power supply, and are partially discharged over the operating range of voltages for the power supply during periods of time when input power is not present.
The energy ES stored in a capacitor (whose capacitance does not change substantially with voltage) is proportional to its capacitance C and to the square of the capacitor voltage V:ES=0.5·C·V2 
If such a capacitor is discharged from an initial voltage VB to a final voltage VA, the energy removed from the capacitor ER is given by the equation:ER=0.5·C·(VB2−VA2)
Thus, the energy available for holdover is dependent on the range of voltage over which it is discharged.
Capacitors generally are not a volumetrically efficient means for energy storage. Among the various capacitor types, aluminum electrolytic capacitors generally are among the best in terms of stored energy density that can be efficiently withdrawn over relatively short intervals of time. The consequence of their low energy density is that a significant number of capacitors is required for the holdover function if there is a substantial power level such as 200 watts and a substantial holdover time such as 5 ms. For an application, for example, such as one with a 43-volt lower nominal input voltage limit and a short-term cut-off voltage of 36 volts, the required capacitance deduced from the equation above would be about 5600 microfarads, considering the typical 20% capacitance tolerance for aluminum electrolytic capacitors, discharged from an initial voltage of 43 volts, and including a voltage loss of perhaps 1.3 volts in isolation diodes that might be required in a practical circuit arrangement. This capacitance could be implemented with 56 capacitors, each rated at 100 microfarads, which would consume a substantial amount of board space, incur significant expense in fabricating a 200 watt power converter, and introduce reliability issues due the large number of additional circuit components required in the product design.
Various circuit arrangements have been used to implement a holdover scheme using a holdover capacitor coupled across the input to a power supply. An example of a prior art circuit as illustrated in FIG. 1 showing a load powered from an input voltage source 101 of 48volts with return current coupled to ground at the terminal gnd. A holdover capacitor CH is coupled across the input to the load through the switch Q100. The load might be required to operate, for example, over a normal input voltage range of 43 to 72 volts and to operate for short periods, 5 ms., for example, down to an input voltage of 36 volts. The holdover circuit might be required to provide an input voltage to the load falling between the upper and lower voltage limits even if the input voltage “drops out” to zero volts for the 5 ms interval.
The holdover capacitor CH is pre-charged by a boost switching regulator to a voltage at the upper end of the normal operating voltage range for the load 120, and is coupled to the load during brief power interruptions by the switch Q100, a p-channel FET. When the load voltage falls below a threshold voltage detected by the comparator 104 using the resistor divider formed by the resistors R4 and R5 and the reference voltage supplied from the VREF terminal of the PWM controller, then the switch Q100 is enabled to conduct, and energy stored in the capacitor CH is supplied to the load. The resistor R6 is included to limit the current surge that results from unequal initial voltages across the capacitors CH and CL when the switch Q100 is closed, and also limits in-rush current to the capacitor CH when the circuit is first energized. Other in-rush control arrangements can be used such as the Texas Instruments TPS2393 controller. The holdover capacitor CH is thus able to power the load for a brief interval as it is discharged over the full normal operating voltage range for the load.
The capacitor CL is a by-pass capacitor for the load 120, and provides the initial holdover energy for the load. If the 48-volt source is momentarily disconnected, the voltage supplied to the load decreases as the capacitor CL is discharged.
Under normal operating conditions when the 48-volt source is powering the load, the capacitor CH is charged by the boost switching regulator comprised of the switch Q2, illustrated in the figure as an n-channel FET, the inductor L1, and the diode D1. The PWM (pulse-width modulated) controller 110 generates a controlled duty-cycle signal for the gate of switch Q2 at its GATE terminal through line 111 to regulate its voltage at the node 112 at the desired voltage level. If a semiconductor technology other than, for example, silicon FET technology is used to implement the switch Q2 then a paralleling diode may be added in parallel with the switch as necessary if such diode is not already inherent in the switching device. The resistor divider comprised of resistors R1 and R2 provide the feedback signal to the PWM controller at its FB terminal. The ground terminal for the PWM controller is shown in the figure as the terminal GND. Bias voltage is provided to the PWM controller 110 at the VDD terminal through resistor R3, which is by-passed to ground through the capacitor C1. The design of boost switching regulators and PWM controllers is well known in the art, and further details such as feedback-stabilizing components are not illustrated and will not be discussed herein in the interest of brevity.
The circuit illustrated in FIG. 1 can discharge the holdover capacitor substantially over the full operating voltage range of the load and can achieve a modest decrease in the size of the holdover capacitor for a required holdover period and load level compared to a scheme in which the holdover capacitor is coupled directly across the input voltage source for the load. Nonetheless, it still has limitations because the voltage of the capacitor CH, which is directly coupled to the load when it is discharged and which provides the principal portion of the holdover energy, cannot be charged to a voltage higher than the specified operating range for the load. Additionally, a significant fraction of the stored energy in the capacitor is not removed during its discharge. Further, the voltage to which the holdover capacitor is charged cannot be selected to take advantage of the voltage rating of commercially available capacitors. The circuit in FIG. 1 can transfer energy from the low voltage capacitor CL to the high voltage capacitor CH, but not in the reverse direction. Thus, there is limited opportunity using the circuit of FIG. 1 to provide an energy storing capacitor with significant energy storage capability without introducing a holdover energy circuit component with substantial volume, cost, and reliability issues.
Another circuit arrangement of the prior art is described in Fraidlin, U.S. Pat. No. 5,258,901, which is hereby referenced and incorporated herein in its entirety. Fraidlin describes a power supply with a diode-rectified ac input voltage source and a boost switching regulator that charges a dc bus to a voltage higher than the peak of the input voltage but within the normal voltage range of the load. Fraidlin uses a switching arrangement to charge the holdover capacitor to the bus voltage when the circuit is operating normally, and when holdover power is required, couples the holdover capacitor across the rectified input voltage to allow the boost switching regulator to discharge the holdover capacitor to a relatively low voltage, thereby removing a substantial portion of the stored energy. Nonetheless, the circuit arrangement described by Fraidlin charges the capacitor to a voltage within the normal voltage range of the load, and still requires a capacitor of substantial volume.
In these prior art circuit arrangements the upper range of the holdover capacitor voltage is limited to the upper voltage limit of the load power converter or any particular load voltage to be held up. It is recognized that the energy storage density of electrolytic capacitors, particularly aluminum electrolytics, is higher for capacitors with a higher voltage rating. However, the capacitance of an electrolytic capacitor of a particular volume generally falls as its voltage rating is increased due to the thicker dielectric isolation layer required between the plates of opposite polarity. Illustrated as an example in FIG. 2, is the capacitance of the TS series of aluminum electrolytic capacitors rated from 16 to 450 volts, produced by Panasonic, all of substantially the same physical volume of approximately 490 cm3. As can be seen in the figure, a substantial decrease in capacitance can be observed as the voltage rating is increased.
However, the energy density of higher-voltage rated capacitors generally increases as their voltage rating is increased. Illustrated in FIG. 3 is the energy storage capability of the same Panasonic TS series of capacitors, all of the same physical volume, again plotted as a function of their voltage rating. As can be seen on the figure, a significant increase in stored energy is obtained, and consequently energy density, for the higher voltage rated aluminum electrolytic capacitors. Modest alternations in the monotonicity of plots of energy density vs. voltage rating for a series of aluminum electrolytic capacitors can sometimes be observed, but the general energy density increase with increasing voltage rating is usually obtained.